1. Field of the Invention
The present invention relates to a process for producing a pulp from a cellulosic starting material using the kraft process.
In particular, the invention relates to a process for producing a high-yield pulp which is suitable for further processing into Viscose fibres and Lyocell fibres.
Dissolving pulps are processed predominantly into cellulosic fibres according to the Viscose and Lyocell process. In recent years, the cost position of regenerated fibres has deteriorated further in comparison to competing synthetic fibres. A reduction in pulp costs, which constitute the most important cost factor, would clearly improve the cost position of regenerated fibres.
2. Description of the Prior Art
Luo et al. have been able to show that conventional kraft pulp is suitable for the production of cellulosic fibres according to the Lyocell process after the average degree of polymerization (DP) has been adjusted via acid hydrolytic and oxidative degradation, respectively [WO 99/47733]. The kraft pulps produced according to said prior art exhibit a content of hemicellulose of at least 7% by weight. According to WO 99/47733, Lyocell fibres produced from those kraft pulps exhibit a hemicellulose content of at least 5 to 27% by weight. However, the measuring method used for determining the hemicellulose content is not disclosed.
However, the presence of low-molecular hemicellulose components has a negative impact on the process behaviour (accumulation of non-fibre-forming hemicellulose portions in the spinning bath; high expenditure of chemicals for the depolymerization for adjusting the desired DP) and the textile-mechanical properties (a strength lower by more than 5 cN/tex under conditioned circumstances in comparison to Lyocell fibres made of prehydrolysis kraft pulps) of the Lyocell fibres produced therefrom.
Since the development of “modified kraft cooking” in the early 80ies it has been known that a decrease in the concentration of dissolved wood ingredients in the final stage of cooking brings about a considerable increase in efficiency and selectivity.
In order to comply with this knowledge, it has been suggested that waste liquor streams rich in hemicellulose, which occur, for example, upon completion of the phase of parallel-flow cooking in continuous cooking processes (e.g.: MCC, EMCC), be withdrawn from the cooking process in order to make sure that the concentration level of the dissolved hemicelluloses drops down to values smaller than 10 g/l in the further cooking phase [WO 95/00698]. In a further step, the high-molecular hemicelluloses are concentrated further by means of pressure-driven membrane separation processes, before they are reprecipitated on the surface of the pulp fibres during final bleaching within the scope of an E-stage in order to increase, on the one hand, the yield, and, on the other hand, the static strength.
This suggestion indeed brings about an improved pulp yield, the disadvantage is, however, that the portion of hemicelluloses which has not been adsorbed (equilibrium reaction) inevitably accumulates in the bleaching filtrate, causes increased consumptions of bleaching chemicals and partly also reaches the sewage.